Do not log an empty error line on tool cancellation

[ptr727]

Problem

When a run is cancelled (SIGTERM), FfProbe operations log an empty "" error line:

[WRN] Operation interrupted : SIGTERM
[ERR] Cancelled execution of FfProbe : ...
[ERR] Failed to get subcc packet info : "/media/.../file.mkv"
[ERR] ""
[ERR] Failed to find Closed Captions in video stream : "/media/.../file.mkv"

FfProbeTool logged the tool stderr as a second error line unconditionally (Log.Error("{Error}", error)) in the subcc, bitrate, and temp-file paths. A cancelled process is killed before writing to stderr, so error is empty and Serilog's {Message} template renders it as "".

This is separate from #821 (which fixed the misplaced quote on the ExitCode line).

Fix

Route the three sites through a LogErrorOutput helper that logs only when the stderr is non-empty (!string.IsNullOrWhiteSpace). Adds tests for the helper (empty/whitespace logs nothing; text logs one Error event).

196 tests pass; formatters/linters clean. No HISTORY entry — the log rework and this fix are both within the unreleased 3.20 cycle.

[Copilot]

Pull request overview

This PR prevents ffprobe cancellation scenarios (e.g., SIGTERM) from producing a redundant empty error log line ([ERR] "") by conditionally logging tool stderr only when it contains non-whitespace text.

Changes:

• Add FfProbe.Tool.LogErrorOutput() helper that suppresses logging when stderr is empty/whitespace.
• Route three existing stderr logging call sites (subcc, bitrate, temp-file creation failure) through the helper.
• Add unit tests validating that empty/whitespace stderr logs nothing, while non-empty stderr logs one Error event.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
PlexCleaner/FfProbeTool.cs	Introduces LogErrorOutput() and uses it to avoid emitting an empty quoted stderr line on cancellation/empty stderr cases.
PlexCleanerTests/FfProbeLogTests.cs	Adds focused tests to assert the helper’s logging behavior for empty vs non-empty stderr strings.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.